Protein A is a 40-60 kDa surface protein originally found in the cell wall of the bacteria Staphylococcus aureus. It has found use in biochemical research because of its ability to bind immunoglobulins. Protein A bind proteins from many mammalian species, most notably IgGs. It binds with the Fc region of immunoglobulins through interaction with the heavy chain. Recombinant Staphylococcal Protein A is often produced in E. coli for use in immunology and other biological research. Protein A is often coupled to other molecules such as a fluorescent dye, enzymes, biotin, colloidal gold or radioactive iodine without affecting the antibody binding site. It is also widely utilized coupled to magnetic, latex and agarose beads. Protein A is often immobilized onto a solid support and used as a reliable method for purifying total IgG from crude protein mixtures such as serum or ascites fluid, or coupled with one of the above markers to detect the presence of antibodies. Immunoprecipitation studies with protein A conjugated to beads are also commonly used to purify proteins or protein complexes indirectly through antibodies against the protein or protein complex of interest. In addition, it is widely used in purification of monoclonal antibodies from wide variety of sources.